Various types of Sagnac interferometer-type fiber-optic current sensors using optical phase modulation are proposed. For example, “All-fiber Sagnac current sensor” shown in FIG. 19 (refer to literature 1), the “Reciprocal reflection interferometer for a fiber-optic Faraday current sensor” shown in FIG. 20 (refer to literature 2), “Production method for a sensor head for optical current sensor” shown in FIG. 21 (refer to National Publication of translated version No. 2005-517961) and “Fiber optic current sensor” shown in FIG. 22 (refer to National Publication of translated version No. 2002-529709) etc. are proposed.
Moreover, “The optical fiber current sensor and its calibration equipment” shown in FIG. 23 (refer to Japanese Laid Open Patent Application No. 2005-345350), and “Fiber optics apparatus and method for accurate current sensing” shown in FIG. 24 (refer to National Publication of translated version No. 2000-515979), etc. propose methods for calculating an electrical current value from the detected light signal by a photodetector other than the conventional examples explained above.
According to the Sagnac interferometer-type fiber-optic current sensors shown in FIGS. 19 to 24, the optical phase modulation with a fixed amplitude and a fixed angular frequency is provided to the light by an optical phase modulator. As the optical phase modulator, a Pockels' cell phase modulator or a piezo-electric phase modulator configured by winding an optical fiber around a cylindrical piezo-electric tube element is used. In addition, the above-mentioned angular frequency is called as a phase modulation angular frequency, and the above-mentioned amplitude is called as a phase modulation depth.    [Literature 1] G. Frosio, H. Hug, R. Dandliker, “All-fiber Sagnac current sensor”, in Opto 92 (ESI Publications, Paris), p 560-564 (April, 1992)    [Literature 2] G. Frosio and R. Dandliker, “Reciprocal reflection interferometer for a fiber-optic Faraday current sensor”, Appl. Opt. 33, p 6111-6122 (September, 1994)
By the way, in the Sagnac interferometer-type fiber-optic current sensors shown in above FIGS. 19-24, the Pockels' cell type optical phase modulator or the piezo-electricity type phase modulator constituted by winding an optical fiber around the cylinder piezo-electricity element is used as the phase modulator. In either phase modulators, the light is phase-modulated by applying a voltage signal of the phase modulation angular frequency to the Pockels' cell element or the cylindrical piezo-electric tube element. Since the phase modulation depth at the time of the phase modulation is adjusted by the magnitude of the amplitude of the above-mentioned voltage signal, the phase modulation depth actually applied to the light is dealt with as proportional to the amplitude of the voltage signal applied to the phase modulator.
However, the phase modulator has temperature characteristics, and the phase modulation efficiency also changes according to the phase modulator's surrounding environmental temperature. Therefore, even if the amplitude of the voltage signal applied to the phase modulator is controlled at a set value, the phase modulation depth actually applied to the light varies. Moreover, the change of the phase modulation efficiency by such phase modulators is also caused by the degradation of the phase modulator itself besides the temperature change.
As a result, since even if the phase modulator is driven with a fixed phase modulation depth, the phase modulation actually applied changes. Accordingly, the change of the modulation depth results in a variation of the sensing output of the current sensor. In both phase modulators of the Pockels' cell phase modulator and the piezo-electric tube phase modulator as mentioned above, the light is phase-modulated by applying a voltage signal of the phase modulation angular frequency to the Pockels' cell element or the cylindrical piezo-electric tube element. Therefore, when a noise is overlapped on the voltage applied to the phase modulator, the sensing output of the Sagnac interferometer-type fiber-optic current sensor is also changed similarly.
In such conventional modulation method, even if the magnitude of the phase modulation actually applied to the light changes, the modulation system is not equipped with a feedback system at all by which the feedback operation is adjusted by detecting the change of the magnitude.
Furthermore, if a polarization extinction ratio in a propagation path of light, especially between a phase modulator and a quarter-wave plate changes, it causes a problem that the sensing output of the Sagnac interferometer-type fiber-optic current sensor changes, and the measurement accuracy is also reduced. For example, in the Sagnac interferometer-type fiber-optic current sensor shown in FIGS. 19 to 24, if the extinction ratio between the phase modulator and the quarter-wave plate deteriorates, the sensing output of the Sagnac interferometer-type fiber-optic current sensor changes. Accordingly, the measurement accuracy reduces.
Moreover, in the Sagnac interferometer-type fiber-optic current sensor proposed in FIG. 19 and FIG. 20, it is assumed that the phase modulator is optically connected with the quarter-wave plate by a polarization maintaining fiber. When mechanical stress, (the stress caused by vibration, sound, or a temperature change is included), is applied to the polarization maintaining fiber, a crosstalk occurs between the lights which propagate along two optic axes of the polarization maintaining fiber, and the polarization extinction ratio changes. Therefore, the sensing output of the Sagnac interferometer-type fiber-optic current sensor also changes.
There are various factors by which the mechanical stress is applied to the above polarization maintaining fiber. For example, people trample the polarization maintaining fiber, and stress may be applied to the fiber. Furthermore, when pulling out the polarization maintaining fiber from a case which causes the quarter-wave plate and a sensing fiber, the pulled out portion of the polarization maintaining fiber from the case is sealed by solder or bonding agent etc. in order to improve the sealing characteristics of the case. When the temperature change occurs at the sealed portion, stress is applied to the polarization maintaining fiber due to the difference in the coefficient of thermal expansion of the materials.
Furthermore, when accommodating the polarization maintaining fiber to optically connect between the phase modulator and the quarter-wave plate by winding in the shape of a coil, or when accommodating the polarization maintaining fiber in a protective tube, the stress is applied to the polarization maintaining fiber due to vibration or acoustic resonance. In this case, the polarization extinction ratio may change, and the sensing output of the Sagnac interferometer-type fiber-optic current sensor may also change.
Moreover, when the phase modulator is optically connected with the quarter-wave plate by the polarization maintaining fiber using an optical connector, the optical connector serves as mechanical connection. Accordingly, optic axis shift between the optically connected polarization maintaining fibers results in a crosstalk and a decrease in the polarization extinction ratio. Even if the polarization maintaining fibers are ideally connected by the optical connector, since the optical connector serves as mechanical connector as above-mentioned, the vibration and temperature change are applied to the optical connector. Accordingly, the optical axis shift may occur between the polarization maintaining fibers to be optically connected. Therefore, it is difficult to keep the polarization extinction ratio stable between the phase modulator and the quarter-wave plate.
Furthermore, when the polarization maintaining fiber between the phase modulator and the quarter-wave plate is optically connected by a fusion splice method using electric discharge, a slight optical axis shift may occur between the polarization maintaining fibers similarly. Therefore, when the polarization maintaining fiber (light transmitting fiber) to connect the sensor head having the quarter-wave plate with a signal processing unit having the phase modulator is separated once at a center portion in the Sagnac interferometer-type fiber-optic current sensor, and the polarization maintaining fibers are optically connected again, the polarization extinction ratio may change. Therefore, the sensed output of the optical current sensor may change, that is, sensitivity changes between before and after the separation of the above-mentioned polarization maintaining fiber.
Thus, the deterioration of the polarization extinction ratio may take place in any portions of the polarizer, the phase modulator and the polarization maintaining fiber to optically connect the optical components, and results in change of the sensing output of the optical current sensor.
Furthermore, when the signal processing methods proposed in FIG. 19, FIG. 21, FIG. 23, and FIG. 24 are used, and the polarization extinction ratio between the phase modulator and the quarter-wave plate changes, the sensed output of the Sagnac interferometer-type fiber-optic current sensor changes. Therefore, the linearity of the input-and-output characteristic deteriorates, and the measurement accuracy also falls due to the change of the sensed output of the current sensor.
In addition, according to the signal processing method shown in FIG. 24, a system is adopted in which one optical phase difference of the two phase modulators is offset by the other phase modulator, and a current value is calculated from the offset phase amount (practically, magnitude of the voltage signal specifically applied to the phase modulator). The detection system is generally called a Serrodyne detection system. In this case, more than two phase modulators are required, and unless the phase modulation efficiency of two phase modulators is not the same, it is difficult to measure correctly the offset phase amount from the voltage applied to the phase modulator. Therefore, the conventional method results in problems that the measurement accuracy falls, the cost is raised due to the use of two or more phase modulators, and the reliability of the sensor falls due to the increase in the number of parts used.